WASHINGTON — The Supreme Court, significantly expanding the rights of poor criminal defendants, ruled today that an indigent suspect must be provided with a psychiatrist at taxpayer expense if the defendant's sanity is a key issue in the case.

In an 8-1 ruling, the court threw out the conviction and death sentence of Glen Burton Ake for killing an Oklahoma minister and his wife in 1979.

The court said the risk of an inaccurate resolution of Ake's insanity plea was magnified because he was denied a psychiatrist's help.

'Risk . . . Extremely High'

"Without the assistance of a psychiatrist to conduct a professional examination on issues relevant to the defense, . . . the risk of an inaccurate resolution of sanity issues is extremely high," said Justice Thurgood Marshall in his opinion for the court.

The impact of the ruling may be limited, however. The court said more than 40 states and the federal government now provide indigent defendants under some circumstances with independent psychiatric help.

The high court rejected arguments by Oklahoma officials that the cost of providing psychiatrists for criminal defendants would be "staggering."

The court said those states that now provide psychiatrists have not found the burden too great when weighed against the rights of defendants.

'Uniquely Compelling'

"The private interest in the accuracy of a criminal proceeding that places an individual's life or liberty at risk is almost uniquely compelling," Marshall said. And, he added, "a state may not legitimately assert an interest in maintenance of a strategic advantage over the defense, if the result . . . is to cast a pall on the accuracy of the verdict obtained."

Justice William H. Rehnquist, in a lone dissent, said the court's ruling "is far too broad."

Rehnquist said providing such help should be limited to cases in which the defendant faces a death sentence. Also, he said, a psychiatrist should be provided only for an independent evaluation, not as a defense consultant.

In Ake's case, Rehnquist added, there was no question from the evidence that he was sane "unless one were to adopt the dubious doctrine that no one in his right mind would commit a murder."